css/css-scroll-anchoring/basic.html

<!DOCTYPE html>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>

body { height: 4000px; }
div { height: 100px; }

</style>
<div id="block1">abc</div>
<div id="block2">def</div>
<script>

// Tests that growing an element above the viewport produces a scroll
// anchoring adjustment equal to the amount by which it grew.

test(() => {
  document.scrollingElement.scrollTop = 150;
  document.querySelector("#block1").style.height = "200px";
  assert_equals(document.scrollingElement.scrollTop, 250);
}, "Minimal scroll anchoring example.");

</script>